Our amigurumi dolls usually have a big head with a small neck, that’s why the head will be floppy if we just stuff the neck with fiber fill.
To avoid a wobbly head, I usually insert an extra arm inside the doll’s neck. The third arm is stuffed inside the neck will make the doll’s neck stronger. It will not make the head stable like using metal wires, but I think it’s the most safety way if you are going to crochet a doll for kid.
But if it still not enough to hold the head, you can use pipe cleaners (it’s pretty cheap and easy to buy at your local craft store). Make sure you cover the edges with yarn, then insert them in the third arm inside the neck.
Here’s the video how I prevent wobbly head syndrome of my amigurumi dolls. I hope it will work with you too!